1. Field of the Invention
The present invention relates to a time information obtaining device and a radio-controlled timepiece including the above time information obtaining device.
2. Description of the Related Art
Conventionally, there is an electronic timepiece (radio-controlled timepiece) which includes a function to receive a standard radio wave which is a radio wave in a long wavelength band to transmit time information and which deciphers the time data to calibrate the present time. In such radio-controlled timepiece, the radio wave is received and the time is calibrated automatically at a predetermined cycle (for example, once a day). Therefore, it is possible to maintain the accurate time display without the user manually calibrating the time.
As the standard radio wave, in addition to JJY (40 kHz, 60 kHz) transmitted in Japan, WWVB (60 kHz) of the United States, MSF (60 kHz) of the United Kingdom, DCF77 (77.5 kHz) of Germany and the like are used. In such standard radio wave, the signal showing the time information is amplitude modulated, and the signal is output each minute at a cycle (frame) of 60 seconds. The signal showing the time information is composed of a plurality of codes arranged according to a predetermined format determined for each standard radio wave transmitting station. The signals showing the codes are output one for each second synchronized with the timing of the beginning of each second. In the radio-controlled timepiece, after the signal is demodulated from the received standard radio wave, the signal is decoded according to the format of the standard radio wave to obtain information of the date and time.
The radio wave in the long wavelength band is transmitted long distances. Therefore, it is possible to receive the standard radio wave throughout a wide range. However, when noise is generated in various places, the noise overlaps with the standard radio wave and is received with the standard radio wave. Moreover, the radio wave attenuates inside a building made of a steel frame or reinforced concrete. Therefore, various techniques are employed to enhance lack of reception intensity and to remove influence of noise so as to obtain time data accurately and securely.
As one of such techniques, typically, the radio-controlled timepiece performs processing to obtain a plurality of pieces of time information from the code string of the plurality of frames and to confirm the consistency among the obtained pieces of time information. Japanese Patent Application Laid-Open Publication No. H11-304973 discloses a technique which divides each frame into blocks showing year, date, day of week, hour, and minute according to a code array format of the transmitting station, and confirms consistency among the plurality of frames of the obtained pieces of time information for each block. For the block in which the frames are not consistent, the standard radio wave is received and the consistency of the time information is confirmed repeatedly until consistency is confirmed.
Alternatively, standard radio waves such as JJY, MSF, and DCF77 include parity data in the time information, and the consistency between the deciphered time data and the parity data can be confirmed.
However, when there is relatively a lot of noise or the radio wave reception intensity is not strong enough, a misidentified code may be mixed in the code string of the frame, and it is not possible to obtain accurate time information for a plurality of frames. In view of the above, the inventors of the present invention developed a method to decide the plausible code by majority of the codes in the same position of the code string determined and obtained for a plurality of frames. However, when the above method is used, since the value of the code regarding one minute unit digit data and the code regarding the parity data including the above code changes for each frame, the code cannot be decided by the majority. Therefore, it is not possible to confirm the accuracy of the time information by consistency of the parity value.
The present invention is a time information obtaining device and a radio-controlled timepiece which can confirm accuracy of time information more easily and more effectively.